This invention pertains to a process for separating phenols from spent aqueous liquors derived from the alkaline pulping of lignocellulose. It pertains particularly to a process for separating phenols from the alkaline black liquors derived from pulping wood by the kraft and soda processes.
As used herein, the term "phenols" is used in its generic sense and pertains not only to phenol itself, but also to phenolic-type compounds generally, and in particular to phenol, orthocresol, meta- and para-cresols, guaiacol, vanillin and acetovanillone.
With the exception of the last named, all of the foregoing phenols have known and important industrial uses. Phenol itself is widely used in plastics, plasticizers, resins, adhesives, fibers, medicinals, herbicides and biocides. The cresols are used in making herbicides, resins, plasticizers, disinfectants, antioxidants and in ore flotation. Guaiacol is used in medicinals and for making perfumes and flavoring materials. Vanillin is well known as a flavoring agent and is used in making medicines.
It is well known that during the alkaline pulping of lignocellulose, phenols of the class described above are produced in substantial quantities by the breakdown of the lignin content of the lignocellulose. For example, a typical sample of kraft black liquor contains 0.85% by weight on a solids basis of such materials. The spent aqueous liquors derived from the alkaline pulping of lignocellulose thus represent a potential source of enormous quantities of phenolic materials. A typical 1,000 tons a day kraft pulp mill, for example, produces from 1550 to 1900 tons of black liquor solids per day. The phenolic content of this quantity of black liquor solids is of the order of from 21,500 to 37,500 pounds.
Up to the present time, there has been no successful attempt to recover commercially the large quantities of phenolic compounds present in this source. These materials presently remain in the black liquor and are burned in the mill recovery cycle.
The reason for this waste of important industrial chemicals is that heretofore no practical, one-step method has been available for isolating them in the small percentage quantities in which they are present from the huge volumes of alkaline liquor in which they are contained.
U.S. Pat. Nos. 2,104,701, 2,489,200, 2,721,221 and 2,871,270 disclose procedures for producing vanillin from waste acid sulfite pulping liquor. However, these procedures require first neutralizing the liquor, which is on the acid side, to produce an alkaline liquor; then oxygenating the alkaline liquor to convert part of its lignosulfonic acid content to vanillin; and then separating the vanillin from the oxygenated liquor.
U.S. Pat. No. 3,375,283 discloses a process for the preparation of methoxyphenols from spent kraft pulping liquor, but by first drying the liquor to a solid and then pyrolizing the resulting solid at 300.degree.-600.degree. C. to generate the methoxyphenols which are separated from the pyrolysate by extraction with benzene.
Chernovsov et al Tr. Vies. Nauch.--Issled Iust. Tsellyul. -- Bum. Prom. 1972, No. 61, pp. 172-80 (Chemical Abstracts, Vol. 80, p. 5089.times.(1974) ) disclose a procedure for isolating from kraft black liquor an extract one component of which is guaiacol. The process consists of: (1) acidifying the alkaline liquor with concentrated hydrochloric acid; (2) heating the liquor to 70.degree. C.; (3) filtering to remove lignin; (4) saturating with sodium chloride; and (5) extracting with diethyl ether. However, the yield of guaiacol is poor and the guaiacol product is heavily contaminated by non-phenolic components from which it can only be extracted by a time-consuming and difficult supplemental procedure.
We now have discovered, and it is the essence of this invention, that the phenolic content of spent aqueous liquors derived from the alkaline pulping of lignocellulose may be separated from the liquors without acidification by the simple expedient of extracting them from the alkaline liquor with a solvent comprising essentially a lower aliphatic alcohol having from 2-5 carbon atoms inclusive, in particular with a propyl alcohol. When this is done, the alcohol solvent extracts the phenolic compounds in the form of their sodium salts, selectively and in high yield. This is accomplished without the necessity of subjecting the liquors to any chemical pretreatment preliminary to the extraction. In addition, it leaves the black liquor in a condition suitable for return to the recovery system of the pulp mill.